1. Field of the Invention
The present invention relates to a hollow fiber membrane module, a hollow fiber membrane module unit, a membrane filtration device (membrane filtration system) used for the purification of potable water supply, purification of waste water, industrial processes or the like, and to a method of operating the same.
2. Description of Related Art
Filtration of water by means of a membrane module has been employed in various applications due to its high separation performance, compact construction of the device and large-quantity and continuous processing capability.
There are different types of membrane modules such as microfiltration modules, ultrafiltration modules, and reverse osmosis modules, which are selected and operated in accordance with the material to be separated. For example, the microfiltration module can efficiently remove fine particles and microorganisms measuring 10 μm or smaller, particularly 1 μm or smaller, and is therefore commonly used in the purification of potable water supplies and waste water.
The microfiltration module is used in the form of a hollow fiber membrane module in which hollow fiber membranes are disposed in a cylindrical configuration or screen configuration, a pleated membrane module in which a flat membrane is folded in pleats in a cylindrical configuration or a planar membrane module in which a flat membrane is disposed in a screen configuration, in order to increase the membrane area and to make it easier to handle.
Among these, the hollow fiber membrane module allows it to provide a large membrane area per unit volume and is preferably used.
Use of the hollow fiber-based microfiltration membrane module in filtration enables it to remove suspended solids and bacteria or the like from water by means of micropores of the membrane and to produce clear filtered water. After a long period of continuous filtering operation, however, the micropores become clogged, resulting in a decrease in flow rate of the filtered water and an increase in the filtration pressure, thus making it necessary to frequently change the membrane module which is economically disadvantageous.
In order to prevent the membrane module from becoming clogged quickly by the membrane clogging matter contained in water, operations to restore the filtering function are carried out periodically such as, in the case of an external pressure-driven hollow fiber membrane module, for example, backwashing wherein filtered water is passed in a reverse direction from the inside to the outside of the hollow fiber membrane, scrubbing cleaning wherein the membrane is vibrated while supplying air to the outside of the hollow fiber membrane, or a combination of these cleaning operations is carried out, thereby removing the membrane-clogging matter deposited on the outside of the hollow fiber membrane.
While the membrane area of the hollow fiber membrane module can be increased by increasing the number of hollow fiber membranes per unit volume, increasing the membrane area by disposing the hollow fiber membranes bundled in cylindrical configuration makes it difficult to flow the scrubbing air or backwash water for cleaning, resulting in decreasing effect of cleaning. This problem can be avoided by disposing the hollow fiber membranes with equal spacing from each other in the form of sheets, which enables uniform cleaning of the membrane surface and makes it applicable also to the filtration of heavily contaminated water.
The hollow fiber membrane module is also used in large-scale purification plants having processing capacity over 10,000 m3/d, in the case of potable water purification. In such an application, a large number of hollow fiber membrane modules are used to secure a large membrane area. For example, when arranging the hollow fiber membrane modules to form a unit, how close the hollow fiber membrane modules can be arranged to each other is restricted by the sizes and configurations of the anchoring members and the water collection section provided at the ends of the hollow fiber membrane modules.
There has been proposed such a hollow fiber membrane module that satisfies the relationship 100≧A/B≧1.2, where it is assumed that the hollow fiber membranes are anchored while being left open at one end or both ends thereof by anchoring members provided in a housing, the area of a surface of the anchoring member is A on the side where the hollow fiber membrane is exposed from the anchoring member, and the area of an end face of the anchoring member is B on the side where the hollow fiber membrane opens (refer to Japanese Unexamined Patent Publication, First Publication No. H07-178320).
This module is assembled by anchoring sheet-form hollow fiber membranes with anchoring members that are molded in elongated rectangular shape. Since width of the anchoring member on the side where the hollow fiber membrane is exposed is larger than the outer diameter of the water collecting tube and larger than the joint, and the water collecting tubes and other members do not interfere with each other, the modules can be disposed in parallel arrangement so that side faces of the anchoring members make contact with each other. As a result, the entirety of the hollow fiber membranes can be cleaned evenly by scrubbing without significant decrease in the density of the hollow fiber membranes.
Higher density of the hollow fiber membranes and lower processing cost per unit area of the hollow fiber membrane can be achieved when the number of the sheet-form hollow fiber membranes fastened onto one anchoring member is larger. However, the number of the hollow fiber membranes that can be fastened onto one anchoring member that is formed in elongated rectangular shape is limited. On the other hand, there is a problem in that increasing the width of the rectangular anchoring member for the purpose of increasing the number of the hollow fiber membranes that can be fastened causes the withstand pressure to drastically decrease.
There is also known a hollow fiber membrane module that has a constitution of a plurality of sheet-form hollow fiber membranes arranged and fastened by an anchoring member onto the end of a cylindrical housing, so that a proper spacing is secured between the sheet-form hollow fiber membranes so as to be efficiently cleaned while increasing the density of the hollow fiber membranes (for example, refer to Japanese Unexamined Patent Publication, First Publication No. 2000-51670).
In this module, however, since the sheet-form hollow fiber membranes are arranged in the cylindrical housing, the width of the sheet-form hollow fiber membrane-fastened at the end becomes smaller than that at the center of the cylinder resulting in lower density of the hollow fiber membranes.
There is also known a hollow fiber membrane module having a constitution of a bundle of hollow fiber membranes divided into a plurality of segments and are expanded and fastened onto a support body at a position near the center between potting sections on both ends, thereby improving the efficiency of cleaning (for example, refer to Japanese Unexamined Patent Publication, First Publication No. H06-99038).
In this module, however, since the hollow fiber membranes are expanded and fastened onto the support body near the center between potting sections on both ends, density of the hollow fiber membranes becomes lower and it becomes difficult to clean the portion near the potting sections.
When the hollow fiber membrane modules are used in a large-scale purification plant, a large number of hollow fiber membrane modules are installed in a submerging water tank in order to increase the membrane area. The hollow fiber membrane modules are disposed in a configuration made up of multiple columns and multiple rows in order to make efficient use of the space in the submerging water tank.
However, this constitution requires a large space for piping and servicing, resulting in a dead space that cannot be efficiently used. As a result, unnecessarily large quantity of washing water is discharged during periodic cleaning operations, thus resulting in lower ratio of recovering of water.
The present invention has been made to solve the problems described above and an object thereof is to provide a hollow fiber membrane module comprising sheet-form hollow fiber membranes that can be easily cleaned, wherein withstand pressure of the hollow fiber membrane anchoring section does not decrease even when the membrane area is increased, and a hollow fiber membrane module unit having high density of hollow fiber membranes. Another object of the present invention is to provide a membrane filtration device that employs the hollow fiber membrane module unit described above and a method of operating the same.